Let’s Review the Science of Extreme Weather

August 30, 2017

Stanford, Calif. — On Sunday, amid the unfolding disaster in Texas, President Trump tweeted his amazement that Hurricane Harvey was producing unprecedented rainfall and flooding. He shouldn’t have been surprised.

Climate science has repeatedly shown that global warming is increasing the odds of extreme precipitation and storm surge flooding. Refusing to acknowledge this impairs our ability to prepare for future extreme weather and endangers American lives and property.

Although seas have risen and warmed, and the atmosphere now holds more moisture, we can’t yet draw definitive conclusions about the influence of climate change on Hurricane Harvey. Hurricanes are complex events, and the role of historical warming in their development continues to be studied. But it is well established that global warming is already influencing many kinds of extremes, both in the United States and around the world, and it is critical to acknowledge this reality as we prepare for the future.

In particular, recent research shows that weather that falls outside of our historical experience is becoming more likely. For example, my colleagues and I recently found that global warming has already increased the odds of record-setting heat waves across more than 80 percent of the planet where we have reliable observations, and influenced record-setting wet and dry events across half that area.

Meteorologists explain that Hurricane Harvey stalled off the Texas coast because two high-pressure atmospheric masses—huge bookends made out of air—have squeezed it in place, and there haven’t been any high-level currents to help steer it away.

Harvey is yet another of several recent weather disasters marked by such shocking staying power, punishing whole regions for days or weeks on end—and longer. Others include a massive heatwave over Russia and flooding in Pakistan in 2010, the Texas drought of 2011, the California drought that began around the same time and continued through this year, and the flooding last year in Texas’s neighbor to east, Louisiana.

Sluggishness in storms is a big deal, particularly if they’re increasing in frequency. “It turns a garden-variety disaster into a catastrophe,” said Paul Douglas, a broadcast meteorologist and weather entrepreneur. As Harvey stays put, it functions as a firehose that sucks warm water from the Gulf of Mexico and the atmosphere, and dumps it inland. As of this writing, meteorologists predict Harvey will move north-northeast Wednesday, and up into the Mississippi basin Friday.

Just to be clear, climate change itself is no longer up for debate—both in the scientific community and among national governments (with few notable exceptions). Understanding it is relatively simple. Global average temperatures are rising. There’s at least 4 percent more water vapor in the air than 70 years ago. Ice sheets are melting. Seas are swelling. These phenomena are the direct effects of warming, and humans are causing it.

“That’s the part of the science everyone agrees about,” said Adam Sobel, professor of applied physics and math, and Earth science, at Columbia University.

What experts don’t agree on, however, is whether—or how much—manmade climate change is responsible for this meteorological stickiness that’s kept Harvey over South Texas.